1. Field of the Invention
The present invention relates to a class D amplifier.
2. Description of the Related Art
In recent years, amplifiers using a class D amplifying method have become more widespread due to the demands for miniaturization and high efficiency requirements of amplifiers.
A class D amplifying method is a method described as follows: In the first place, modulation processing such as pulse width modulation (PWM) or pulse density modulation (PDM) is performed on an analog signal which is input to an amplifying circuit. Then the digital signal which is converted from the input signal by the above modulation processing is amplified. Next, the amplified signal is passed through a low-pass filter and is converted back to an analog signal. In an amplifier using a class D amplifying method, the analog signal can be amplified only by amplifying the digital signal, that is, only by ON/OFF signal processing. Thus, theoretically, 100% power efficiency can be obtained.
Thus, amplifiers using a class D amplifying method are highly efficient, thereby making miniaturization.
In this regard, as the low-pass filter used for the output from a class D amplifier, an LC-type low-pass filter has been widely used. The LC-type low-pass filter typically comprises passive elements such as an inductor, a capacitor, and so on. Such an LC-type low-pass filter has many advantages, for example, low power loss and stable characteristics.
In designing such an LC-type filter, the design is made on the assumption that a load having a constant impedance, which is called a “nominal impedance”, is connected to the filter. However, the load impedance which is connected to the output of a class D amplifier, in other words, the output of the LC-type filter, may fluctuate. If this impedance value deviates from the nominal impedance, the frequency characteristic of the LC-type filter fluctuates considerably because of the nature of the LC-type filter. Thus, there is a shortcoming in that, the frequency characteristic of the class D amplifier, as a whole, differs from the design value.
Particularly, in the case of a speaker, which is usually used as the load of an amplifier, the following problem occurs: The impedance of the speaker may deviate from its nominal impedance in the high frequency band due to the configuration of the magnetic circuit of a voice coil unit in the speaker, and the characteristic of a network circuit which is inserted at the pre-stage of the speaker.
Accordingly, in a conventional class D amplifier, an impedance compensation circuit, such as a Zobel element, is connected to the load end of the low-pass filter for output in parallel with a load. This means that it has been necessary to compensate the impedance characteristic of a speaker, which is a load, by using such an impedance compensation circuit.
However, with a compensation circuit composed of such passive elements, its compensation ability is limited, and the circuit cannot cope with a wide range variation of the load impedance. Also, with such a compensation circuit, there is a problem in that, once compensation is performed for a specific speaker which is connected as a load, the previously adjusted compensation circuit cannot function when the speaker is replaced with another speaker.